There are available a variety of means for measuring the transient behaviors of ultra-high speed optical phenomena. One of the measuring means employs a streak camera in which an incident light signal is converted into an electron beam that is allowed to sweep at high speed, so that the intensity of an incident light signal that changes with time is measured as a variation in luminance with respect to position on the phosphor screen.
As shown in FIG. 17, an essential component of the streak camera, namely, a streak tube 13 comprises a photocathode 14 for converting into an electron image the light (slit image) which is applied through a slit plate 10 and image formed by a lens 12 of an input optical system; a mesh type accelerating electrode 16 for accelerating the electron image provided by the photocathode 14; deflecting electrodes 22 for deflecting, at high speed, the electron beam accelerated by the accelerating electrode 16 in a direction (which is a vertical direction in FIG. 17) perpendicular to the longitudinal direction of the slit; and a phosphor screen 26 for converting the electron image deflected by the deflecting electrodes 22 into an optical image (i.e.; a streak image that is a luminance data image in which the vertical axis represents the lapse of time).
Further in FIG. 17, a focusing electrode 18 is provided for focusing, to a certain degree, the electron beam accelerated by the accelerating electrode 16. An aperture electrode (or anode) 20 is provided for further accelerating the electron beam. A sweep circuit 23 is provided for applying a predetermined sweep voltage across the deflecting electrodes 22 in synchronism with the passage of the electron beam A micro channel plate (MCP) 24 is provided in front of the phosphor screen 26 to multiply the number of electrons passed through the deflecting electrodes 22. A conical shield electrode 25 is provided on the input side of the MCP 24 for blocking the electrons deflected out of the effective sweep region of the phosphor screen to improve the accuracy of measurement. Additionally, an image pickup device 28 comprising a high sensitivity television camera such as an SIT camera or CCD camera is provided for recording the streak image through a lens 27 of an output optical system.
Generally, the above-described streak camera is classified in a single sweep type streak camera and synchro-scan type streak camera depending on the operating principle employed; i.e., the sweep system employed. In the single sweep type streak camera, a linear sweep is carried out by using an ultra-high speed saw tooth wave up to several kilohertz (KHz) in synchronism with a pulse laser beam. In the synchro-scan type streak camera, a high-speed repetitive sweep is carried out with a sine wave of 80 to 160 MHz in synchronism with a laser beam. In addition to the above-described two types of streak cameras, a synchronous blanking type streak camera has been developed in which an elliptical sweep is carried out. That is, as shown in FIG. 18, the return sweep is shifted sidewards so that the electron beam does not go across the phosphor screen 26.
The above-described method using the streak camera is a pure-electronic direct method having excellent time resolution and detection sensitivity. The method can measure single shot (non-repetitive) phenomena. In addition, for a streak image that is originally two-dimensional, the method can be used for time-resolved spectroscopic measurement or space-and-time-resolved measurement. Further, with the materials of the photocathode and the incident window appropriately selected, the method can perform measurement over a wide range of spectral sensitivity, e.g., ranging over near infrared, vacuum ultraviolet, and X-ray regions.
In addition, a sampling type optical oscilloscope has been put in practical use which, as shown in FIG. 19, has a sampling streak tube 30 in which a slit board 32 is provided for spatially limiting the streak image in the streak camera to electrically sample the streak image
In FIG. 19, reference numeral 34 designates a photodetector for detecting the intensity of light which is emitted by phosphor screen 26 when an electron beam impinges on the latter. The photodetector may be a photomultiplier tube, high sensitivity photodiode, avalanche photodiode, or PIN photodiode.
The above-described streak cameras employ the streak tubes 13 and 30. Therefore, the utility of light is limited to 10 to 20% of maximum depending on the conversion efficiency of the photocathode 14.
On the other hand, an electro-optical deflector 36, as shown in FIG. 20, has recently been developed which deflects a light beam by using the fact that, by the electro-optic effect, the refractive index of a crystal of LiTaO.sub.3, BaTiO.sub.3, KTN or AMO can be changed In FIG. 20, deflector 36 comprises an electro-electro optic crystal 36A and electrodes 36B. The deflector does not employ a photocathode and a light beam applied to the electro-optic crystal 36A is deflected as it is, so that an improvement of the light utility factor can be realized.
The electro-optical deflector 36 utilizes the refractive index change (.DELTA.n) caused by the electro-optic effect. The refractive index change (.DELTA.n) is usually proportional to the internal electric field. The electric field changes spatially linearly within the crystal 36A, and therefore the incident light beam is deflected to form a phase difference spatially. The deflector described above can withstand high voltage, and can be manufactured readily. Further, it can provide a deflection angle about two times that provided by a compound prism type deflector when the two deflectors are configured the same and are operated with the same voltage.
A so-called "electro-optical streak camera" has been developed which, as shown in FIG. 21, employs the above-described electro-optical deflector 36 to directly deflect a light beam under measurement. When, in the electro-optical streak camera, the incident light beam is deflected, in a sweep mode, with the electro-optical deflector 36 and an output lens 38, the image whose vertical axis represents the lapse of time is obtained on the Fourier transform plane of the lens 38. As a result, the variation with time of a light signal can be measured by spatial analysis.
In the electro-optical streak camera, the electro-optical deflector 36 operates at an extremely high speed on the order of picoseconds. Therefore, the streak camera can have a relatively simple construction. In addition, the electro-optical streak camera is substantially unaffected by vibration or electro-magnetic field noise
However, the conventional electro-optical streak camera is still disadvantageous in that, since it includes no function of amplifying the intensity of a light beam under measurement, it is low in sensitivity and therefore can not measure low-light phenomena. It is, therefore, difficult to put the electro-optical streak camera into practical use.
In order to eliminate the above described difficulty, the following solution has been proposed: The incident light beam, which heretofore was only allowed to pass through the central portion of the electro-optical crystal 36A, is passed substantially through the entire cross-sectional area of the electro-optical crystal 36A. That is, all of the light passed through the crystal 36A is utilized, so that the sensitivity is improved with the increased area. However, this solution is not sufficient to eliminate the above-described difficulty.
Further, no suitable detecting method has been proposed for the detection of streak images and, accordingly, it has been difficult to detect streak images with high sensitivity and with high S/N ratio.